Genetics of Brewer's Yeast
Overview Beer is the world's most widely consumed alcoholic beverage (5). Beer is produced by the hydrolysis of starch into a sugar, then the fermentation of that sugar (5). The fermentation process is done by enzymes that are contained within living yeast cells. Enzymes that hydrolyze starch can be found in the malted barley/wheat that gets added to the beer (2). Through the use of genetic technology, such as transformations, recombinant technology and gene expression, certain enzymes can be over-produced to make a desirable beer (4). Beer Making Ingredients and Process There are 5 main ingredients that go into making beer; starch source, yeast, water, hops and a clarifying agent. The starch source, generally wheat or barley, is added to the beer so that it can be saccharified (turned into a sugar, or hydrolyzed), then fermented to produce alcohol (1). The saccharification occurs because of certain enzymes within the wheat. Yeast are added to the beer to ferment the sugar source, which in-turn will produce alcohol within the beer. Hops are added to beer for flavoring mostly, the ratio of certain hops can influence the taste of the beer; such as making the beer bitter or even sweeter. Water is the liquid that these chemical reactions take place in (1). Interestingly, the mineral concentrations within the water can influence the taste of the beer. Certain geographical areas contain water that are more desired for beer making due to the taste. The final ingredient is the clarifying agent, which precipitates undesirable solids, such as enzymes, proteins and yeast cells, and will make the beer clear and appealing. The actual brewing process takes about 3 hours, while the final product is recommended to sit for a minimum of 2 weeks before consuming (5). Yeast Within Beer As was stated earlier, the job of the yeast within beer is ferment the resulting hydrolyzed sugar into alcohol and carbon dioxide (6). There are two widely used yeast strains for fermentation; Saccharomyces cerevisae and Saccharomyces uvarum. ''The process of alcoholic fermentation, or ethanol fermentation, is an anaerobic one; so it occurs in the absence of oxygen (6). The overall chemical process of fermentation is (1): C6H12O6 + Zymase → 2 C2H5OH + 2 CO2 This reaction shows that a sugar, such as glucose, fructose, or sucrose, plus Zymase leads to 2 ethanol molecules and 2 carbon dioxide molecules to be produced. Zymase is an enzyme complex that catalyzes the fermentation reaction. The Zymase enzyme complex is present within these two yeast strains, which make them ideal for brewing (2). Fermentation activity within yeast cells varies between different strains; strains with more Zymase activity will be better suited for brewing. Yeast Genetics The Zymase enzyme complex contains many different enzymes, each with a specific purpose in regards to the fermentation reaction. These enzymes include sucrases and maltases (specific to the sugar source), reductases, carboxylases, and proteolytic enzymes such as an endo-tryptase. These enzymes are encoded for within the yeast strain's genome. Since these enzymes are encoded for within the genome, scientists can influence the gene expression of these products to increase or decrease the relative amount of fermentation that occurs within the cells. By increasing the expression of these genes, more enzymes will be produced, which in-turn means that one cell can increase its fermentation rate compared to a wild-type cell. Influencing the genetics of brewer's yeast has been going since the 1900s through the use of artificial selection, selected breeding, and even transformations. Yeast cells are an ideal candidate for transformation due to their ability to uptake foreign DNA. Certain groups have been conducting experiments on the metabolism of ''S. cerevisiae ''to influence the rate of fermentation. While genetically-modified organisms are still a cause for concern to the public, certain strategies have already been employed by scientists to affect the metabolic rate of fermentation by yeast cells. Overall, if the expression of Zymase enzymes could be increased, it could result in a decrease of brewing time. References 1. Enzymes of Yeast ''Chest of Books - Alcohol Properties of Yeast 2. Genetic improvement of brewer's yeast: current state, perspectives and limits. ''PMID: 20195857 3. ''Brewer's Yeast: Genetics and Biotechnology. ''Julio Palaina 4. ''Brewing A Great Beer: DNA Study Reveals Evolution of Beer Yeasts. ''Science Daily 5. ''Beer. ''Wikipedia 6. ''Ethanol Fermentation. Wikipedia